Solutions of Organic Salts as Pretreatments for Plastic Prior to Etching

ABSTRACT

A method of preparing a plastic substrate to accept metal plating thereon is described. The method includes the steps of pretreating the plastic substrate by contacting the plastic substrate with an aqueous electrolyte comprising an organic salt to raise the surface energy of the plastic substrate. Thereafter, the plastic substrate can be etched and metal plated.

FIELD OF THE INVENTION

The present invention relates generally to an improved method of pretreating a non-conductive substrate in a plastic metallization process.

BACKGROUND OF THE INVENTION

It is well known to plate non-conductive substrates, (i.e. plastics) with metal for a variety of purposes, including for decoration and for the fabrication of electronic devices. An example of a decorative use is for automobile parts such as trim. Examples of electronic uses include printed circuits, wherein metal plated in a selective pattern comprises the conductors of the printed circuit board, and metal plated plastics used for EMI shielding. ABS resins are among the most commonly plated plastics for decorative purposes while phenolic and epoxy resins are among the most commonly plated plastics for the fabrication of printed circuit boards.

There are typically many stages involved in the plating of plastics. A typical processing sequence for preparing plastics for subsequent plating thereon includes the following steps:

(1) etching the substrate with a chromic acid etching solution;

(2) neutralizing the etched surface with a chrome neutralizing solution;

(3) activating the etched surface using a colloidal palladium tin activator;

(4) removing tin with an accelerating step; and

(5) depositing a layer of electroless copper or electroless nickel followed by electrolytic copper and/or nickel plating.

For the durability of metal layers deposited on the plastic substrate surfaces, it is important that the metal layers exhibit sufficient adhesion to the surface. In order to achieve this adhesion, the plastic surface is typically treated by roughening or etching to provide a suitable surface to accept metal plating thereon. Etching the plastic provides mechanical adhesion of the subsequent metallic coatings and provides a suitable surface for the adsorption of the palladium colloid catalyst which is typically applied in order to catalyze deposition of the initial metallic layer from an autocatalytic nickel or copper plating process.

The initial etching of the plastic surfaces is a critical element of the overall process. However, only certain types of plastic components are suitable for plating. One of the most common types of plastic for electroplating is acrylonitrile/butadiene/styrene (ABS) or a blend of this material with polycarbonate (ABS/PC). ABS consists of two phases—a relatively hard phase consisting of an acrylonitrile/styrene copolymer and a softer polybutadiene phase.

Currently, this material is etched almost exclusively using a mixture of chromic and sulfuric acids, which is highly effective as an etchant for both ABS and ABS/PC. The polybutadiene phase of the plastic contains double bonds in the polymer backbone, which are oxidized by the chromic acid, thus causing complete breakdown and dissolution of the polybutadiene phase exposed at the surface of the plastic which gives an effective etch to the surface of the plastic.

However, a significant problem with the traditional chromic acid etching step is that chromic acid is a recognized carcinogen and is increasingly regulated, requiring that wherever possible, the use of chromic acid be replaced with safer alternatives. The use of a chromic acid etchant also has well-known and serious drawbacks, including the toxicity of chromium compounds which makes their disposal difficult, chromic acid residues remaining on the polymer surface that inhibit electroless deposition, and the difficulty of rinsing chromic acid residues from the polymer surface following treatment. Additionally, hot hexavalent chromic acid solutions are naturally hazardous to workers. Burns and upper respiratory bleeding are common in workers routinely involved with these chrome etch solutions.

Early attempts to replace the use of chromic acid to etch plastic typically focused on the use of permanganate ions as an alternative to chromic acid. The use of permanganate was described by U.S. Pat. No. 4,610,895 to Tubergen et al., the subject matter of which is herein incorporated by reference in its entirety. Later, the use of permanganate was described in combination with an ionic palladium activation stage as set forth in U.S. Pat. Pub. No. 2005/0199587 to Bengston, the subject matter of which is herein incorporated by reference in its entirety. The use of permanganate solutions in combination with perhalo ions (such as perchlorate or periodate) was described, for example in U.S. Pat. Pub. No. 2009/0092757 to Satou, the subject matter of which is herein incorporated by reference in its entirety. Finally, International Publication No. WO2009/023628 to Schildman et al., the subject matter of which is herein incorporated by reference in its entirety, described the use of permanganate ions in the absence of alkali metal or alkaline earth metal cations.

However, all of these attempts to etch plastic using permanganate ions were not capable of producing etch characteristics which match those obtained by the use of chromic acid and the stability of these etch solutions was also poor, resulting in the formation of manganese dioxide sludge.

As a result, none of these processes proved satisfactory for various economic, performance and/or environmental reasons and none of these processes have achieved commercial success or been accepted by the industry as suitable replacements for chromic acid etching.

U.S. Pat. Pub. No. 2013/0186774 to Pearson et al., the subject matter of which is herein incorporated by reference in its entirety, describes the use of trivalent manganese in combination with strong sulfuric acid to etch plastics, such as ABS and ABS/PC plastics. As described therein, trivalent manganese can readily be produced by electrolysis at low current density of divalent manganese ions in a strong acid solution, and a solution of trivalent manganese ions in a strongly acidic solution is capable of etching ABS and is a suitable replacement for chromic acid etching solutions. In addition, related U.S. Pat. Pub. No. 2013/0186862 to Pearson et al., the subject matter of which is herein incorporated by reference in its entirety, describes that that it is possible to increase the amount of manganese that can be dissolved in the bath by replacing a portion of the sulfuric acid with another acid, such as methane sulfonic acid in which the manganese ions may be more soluble. The additional acid must have both the necessary stability against oxidation and the ability to increase the solubility of manganese ions. Finally, related U.S. Pat. Pub. No. 2013/0186861 to Pearson et al., the subject matter of which is herein incorporated by reference in its entirety, describes the use of vitreous carbon and lead as electrodes in the system for producing a manganese(III)-based etchant.

One of the differences between the use of these manganese(III)-based etching solutions as compared with the chromic acid etching solutions of the prior art is that it was found that it is necessary in most situations to pre-treat the plastic prior to etching to raise its surface energy. Thus, most processes require the use of a pretreating solvent to raise the surface energy of the surface to a sufficient degree to provide good adhesion of the subsequently applied metal plating layer thereto.

Various solvents have been investigated for modifying the surface of the plastic substrate and raise its surface energy, including, for example, butyrolactone and diglyme. However, butyrolactone is a controlled substance due to its tendency to hydrolyze to gamma hydroxybutyric acid which is a controlled drug, and diglyme is also heavily regulated due to its mutagenic and hepatotoxic properties. Another issue with the use of these materials is that they are only marginally effective on some moldings and grades of ABS. Based thereon, additional research is needed to provide new pretreatment compositions that are capable of providing a good result and that are less toxic than the solvents previously used.

Thus, there remains a need in the art for an improved method of preparing a plastic substrate to accept metal plating thereon that overcomes the deficiencies of the prior art.

SUMMARY OF THE INVENTION

It is an object of the present invention to improve adhesion of a metal layer plated on a plastic substrate.

It is another object of the present invention to improve the adhesion of a metal layer plated on an ABS or ABS/PC substrate.

It is another object of the present invention to provide an improved method of preparing a plastic substrate to accept metal plating thereon.

It is still another object of the present invention to provide an improved method of pre-treating a plastic substrate that is compatible with chrome and chrome-free etchants.

To that end, in one embodiment, the present invention relates generally to a method of treating a plastic substrate to accept metal plating thereon, the method comprising the steps of:

a) pretreating the plastic substrate by contacting the plastic substrate with an aqueous electrolyte comprising an organic salt; and thereafter

b) etching the treated plastic substrate with an etchant.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates generally to a method of treating a plastic substrate to accept metal plating thereon, the method comprising the steps of:

a) pretreating the plastic substrate by contacting the plastic substrate with an aqueous electrolyte comprising an organic salt; and thereafter

b) etching the treated plastic substrate with an etchant.

The inventors of the present invention have discovered that if ionic liquids, comprising an organic salt having a melting point of less than 100° C. at 100 bar, are diluted in a suitable solvent, the ionic liquid dissociates into an aqueous solution of the corresponding organic salt in solution. This aqueous electrolyte based on the ionic liquid is compatible with parts which are subsequently to be processed with a chrome or chrome-free etchant and is capable of raising the surface energy of the underlying plastic substrate so as to allow for the plastic substrate to be adequately treated with a chrome or chrome-free etchant. Chrome-free etchant means that no chrome containing compounds are intentionally added to the etch solution and chrome concentration due to contaminants in the etch solution is less than 1 ppm.

By definition, ionic liquids are salts that melt at low temperatures (typically less than about 100° C.) and which are made up exclusively of ions. In other words, an ionic liquid is a liquid salt that consists of ions and ion pairs.

Ionic liquids are typically composed of heterocyclic organic cations and various anions and have unique properties including non-volatility, non-flammability, and a wide temperature range for the liquid phase. The molecular weight of most ionic liquids is typically less than about 2000 g/mol, and may be less than about 1500 g/mol, or even less than about 750 g/mol.

In one embodiment, the compositions of the present invention may be based on imidazolium compounds. Particularly preferably imidazolium compounds include imidazolium compounds of the formula:

wherein R1 and R3 are each, independently of one another, an organic radical having from 1 to 20 carbon atoms, R2, R4 and R5 are each, independently of one another, an H atom or an organic radical having from 1 to 20 carbon atoms, X is an anion and n is 1, 2 or 3.

Examples of imidazolium compounds include, for example, 1,3-dimethylimidazolium methylsulfate, 1,3-dimethylimidazolium hydrogensulfate, 1,3-dimethylimidazolium dimethylphosphate, 1,3-dimethylimidazolium acetate, 1-ethyl-3-methylimidazolium methylsulfate, 1-ethyl-3-methylimidazolium hydrogensulfate, 1-ethyl-3-methylimidazolium thiocyanate, 1-ethyl-3-methylimidazolium acetate, 1-ethyl-3-methylimidazolium methanesulfonate, 1-ethyl-3-methylimidazolium diethylphosphate, 1-(1-butyl)-3-methylimidazolium methylsulfate, 1-(1-butyl)-3-methylimidazolium hydrogensulfate, 1-(1-butyl)-3-methylimidazolium thiocyanate, 1-(1-butyl)-3-methylimidazolium acetate, 1-(1-butyl)-3-methylimidazolium methanesulfonate, 1-(1-dodecyl)-3-methylimidazolium methylsul fate, 1-(1-dodecyl)-3-methylimidazolium hydrogensulfate, 1-(1-tetradecyl)-3-methylimidazolium methylsulfate, 1-(1-tetradecyl)-3-methylimidazolium hydrogensulfate, 1-(1-hexadecyl)-3-methylimidazolium methylsulfate or 1-(1-hexadecyl)-3-methylimidazolium hydrogensulfate, and combinations of one or more of the foregoing.

In another embodiment, the aqueous electrolyte may contain ionic liquids such as 2-hydroxyethylammonium formate or methyltributylammonium methylsulfate (MTBS) alone or in combination with imidazolium compounds.

Particularly preferred examples, include, but are not limited to 1-ethyl-2-methylimidazolium acetate and methyltributylammonium methylsulfate (MTBS).

In the process described herein, the ionic liquid or similar organic salt is diluted with water or any non-ionic solvent that is able to decompose the ionic liquid and produce an aqueous electrolyte comprising an organic salt without having deleterious effects on the substrate. Thus, the present invention is not directed to an ionic liquid for pre treating the plastic substrate but is instead directed to the use of an aqueous electrolyte that contains an organic salt. As used herein an organic salt is the ions that result from mixing an ionic liquid with a solvent such as water.

The composition of the present invention may further comprise additional solvents, pH adjusters, buffers, thickeners, co-solvents such as glycerol, propylene carbonate, isopropanol and glycol ethers, and surface active agents for modifying the surface tension of the solution, by way of example and not limitation.

As described herein, the aqueous electrolyte solutions of the present invention are prepared by diluting an ionic liquid or other suitable organic salt with a solvent that is capable of decomposing the organic salt. In a preferred embodiment, the composition comprises less than 50% by weight of the organic salt, more preferably less than 30% by weight of the organic salt diluted with water, and most preferably less than about 20% by weight of the organic salt. In one embodiment, the composition described herein includes about 10 to about 20% by weight of the organic salt in an aqueous solution of water and/or another suitable solvent.

Prior to contacting the plastic substrate with the aqueous electrolyte composition described herein, the substrate may be cleaned and degreased.

Thereafter, the cleaned and degreased plastic substrate is contacted with the aqueous electrolyte of the invention. The contacting time of the plastic substrate with the aqueous electrolyte as well as the contacting temperature (i.e. the temperature at which the aqueous electrolyte is maintained) will vary depending on the substrate being treated and the composition of the aqueous electrolyte.

In a preferred embodiment, the plastic substrate is contacted with the aqueous electrolyte at a temperature of between 0° C. and about 120° C., more preferably at a temperature of between about 40° C. to about 100° C. The aqueous electrolyte can be contacted with the plastic substrate by various means including immersion or spraying. In a preferred embodiment, the plastic substrate is contacted with the aqueous electrolyte by immersing the plastic substrate in the aqueous electrolyte.

The plastic substrate is contacted with the aqueous electrolyte for a period of time sufficient to raise the surface energy of the plastic substrate. Thus the contacting time may be between about 30 seconds and about 10 minutes, more preferably between about 1 minute and about 5 minutes. The surface energy of ABS test panels was found to be 30-32 dynes/cm without the use of the current invention prior to etching and when treated for 5 minutes at 50° C. using an aqueous electrolyte of the current invention on ABS test panels prior to etching, the surface energy was increased to 32-36 dynes/cm.

After being contacted with the aqueous electrolyte, the plastic substrate is optionally but preferably rinsed to remove any aqueous electrolyte remaining on the surface. The rinsing step is preferably carried out using a water rinse. The rinsing step may preferably be carried out for between about 30 seconds and about 5 minutes, more preferably for between about 1 minute to about 3 minutes and may be accomplished by spraying or immersion.

Once the plastic substrate has been contacted with the aqueous electrolyte described herein under suitable conditions to raise the surface energy of the plastic substrate and then rinsed, it may then be contacted with an etchant as described for example in U.S. Pat. Pub. No. 2013/0186774 to Pearson et al., the subject matter of which is herein incorporated by reference in its entirety.

Examples of the current invention as compared to prior art examples are shown below. All chemistry used to carry out these experiments is available from MacDermid Inc.

Comparative Example 1 Chrome Etch

Alkaline cleaner (2 minutes, 60 C)

Rinse

Chrome etch (9 minutes, 68 C)

Rinse

Chrome Neutraliser (Macuplex 9338, 2 minutes, 50 C)

Rinse

Acid Dip (Hydrochloric acid, 2 minutes at room temperature) Activator (Mactivate 360, 3 minutes, 30 C)

Rinse

Accelerator (Ultacel 9369, 2 minutes, 50 C)

Rinse

Electroless nickel (Macuplex J64, 7 minutes, 30 C)

Rinse

Acid Copper plate (Cumac Optima, 90 minutes, 3.5 A/dm2)

Rinse and dry

Peel strength tested according to ASTM B533 using an Instron Peel Test instrument.=9.8 N/cm

Example 1 Pretreatment Using the Current Invention Prior to Chrome Etch

Alkaline cleaner (2 minutes, 60 C)

Rinse Pretreatment (200 g/1 MTBS, 5 Minutes, 50 C) Rinse

Chrome etch (9 minutes, 68 C)

Rinse

Chrome Neutraliser (Macuplex 9338, 2 minutes, 50 C)

Rinse

Acid Dip (Hydrochloric acid, 2 minutes at room temperature) Activator (Mactivate 360, 3 minutes, 30 C)

Rinse

Accelerator (Ultacel 9369, 2 minutes, 50 C)

Rinse

Electroless nickel (Macuplex J64, 7 minutes, 30 C)

Rinse

Acid Copper plate (Cumac Optima, 90 minutes, 3.5 A/dm2)

Rinse and dry

Peel strength tested according to ASTM B533 using an Instron Peel Test instrument.=14.6 N/cm

Comparative Example 2 Chrome-Free Etch

Alkaline cleaner (2 minutes, 60 C)

Rinse Pre-Treatment (10% v/v Propylene Carbonate, 5% v/v Gammabutyrolactone in Water, 2 Minutes at 35 C) Rinse

Manganese(III) etch solution (15 minutes, 68 C)

Rinse

Chrome Neutraliser (Macuplex 9338, 2 minutes, 50 C)

Rinse

Acid Dip (Hydrochloric acid, 2 minutes at room temperature) Activator (Mactivate 360, 3 minutes, 30 C)

Rinse

Accelerator (Ultacel 9369, 2 minutes, 50 C)

Rinse

Electroless nickel (Macuplex J64, 7 minutes, 30 C)

Rinse

Acid Copper plate (Cumac Optima, 90 minutes, 3.5 A/dm2)

Rinse and dry

Peel strength tested according to ASTM B533 using an Instron Peel Test instrument.=1.9 N/cm

Example 2 Pretreatment Using the Current Invention Prior to Chrome-Free Etch

Alkaline cleaner (2 minutes, 60 C)

Rinse Pretreatment (500 g/l MTBS, 5 Minutes, 50 C) Rinse

Manganese(III) etch solution 15 minutes at 68 C

Rinse

Chrome Neutraliser (Macuplex 9338, 2 minutes, 50 C)

Rinse

Acid Dip (Hydrochloric acid 2 minutes at room temperature) Activator (Mactivate 360, 3 minutes at 30 C)

Rinse

Accelerator (Ultacel 9369, 2 minutes, 50 C)

Rinse

Electroless nickel (Macuplex J64, 7 minutes, 30 C)

Rinse

Acid Copper plate (Cumac Optima, 90 minutes, 3.5 A/dm2)

Rinse and dry

Peel strength tested according to ASTM B533 using an Instron Peel Test instrument.=6.0 N/cm

Example 3 Pretreatment Using the Current Invention with Chrome-Free Etch

Alkaline cleaner (2 minutes, 60 C)

Rinse Pretreatment (200 g/1 MTBS, 5 Minutes, 50 C) Rinse

Manganese(III) etch solution 15 minutes at 68 C

Rinse

Chrome Neutraliser (Macuplex 9338, 2 minutes, 50 C)

Rinse

Acid Dip (Hydrochloric acid 2 minutes at room temperature) Activator (Mactivate 360, 3 minutes at 30 C)

Rinse

Accelerator (Ultacel 9369, 2 minutes, 50 C)

Rinse

Electroless nickel (Macuplex J64, 7 minutes, 30 C)

Rinse

Acid Copper plate (Cumac Optima, 90 minutes, 3.5 A/dm2)

Rinse and dry

Peel strength tested according to ASTM B533 using an Instron Peel Test instrument.=7.3 N/cm

This set of experiments was carried out using increasingly dilute ionic liquid. Surprisingly, it was found that even at a dilution of 20% of the ionic liquid in water, excellent adhesion was obtained following etching in both chrome and chrome-free etchant solutions. In each case an ABS substrate was immersed in an aqueous solution containing the ions of an organic salt for 5 minutes at 50° C. The substrates were then briefly rinsed and then etched in an etchant. The substrates were then electrolessly plated with copper, in each case, the adhesion of the metal plates was excellent.

The act of diluting the ionic liquid converted the ionic liquid into an aqueous electrolyte solution of an organic salt in water, so it is no longer an ionic liquid. The transition between an ionic liquid and a solution of an organic salt in water depends on the solvation number for the ions in solution. The generally accepted definition of an ionic liquid is simply an ionic salt consisting only of ions above its melting point. In this state, the ions which were held in the crystal lattice become mobile.

If we take sodium chloride as an example, it becomes an ionic liquid above its melting point of 801° C. At room temperature, a 5.35M solution in water is readily obtained. However, this would never be described as an ionic liquid. At this concentration, the solution contains 260 g/kg of sodium chloride so 1 kg of this solution would contain 740 g/kg of water or 41.1 moles of water and 4.44 moles of sodium chloride (8.88 moles of ions). It can be inferred from that in this case, every ion will have 9 molecules of water available for solvation. Of course, in the case of an organic ionic liquid, the solvation number will be different, but is likely to be lower than 9 because larger ions are less polarizing than smaller ones.

If we consider methylttibutylammonium methylsulfate (MTBS) which has a molecular mass of 317, it is simple to calculate the amount of molecules of water available per ion versus concentration. At a concentration of about 50% MTBS, there were will be more than 10 molecules of water available per ion of MTBS. This would certainly be considered to be an electrolyte solution, and not an ionic liquid. Above this concentration, the interpretation is somewhat less clear-cut, but almost certainly any concentration of MTBS below 70% (4 molecules of water per ion) would be considered to be an aqueous electrolyte solution rather than an ionic liquid.

As described herein, it has been unexpectedly found that ionic liquids that are sufficiently diluted to produce an aqueous electrolyte containing an organic salt can be beneficially used to pretreat an ABS or ABS/PC plastic substrate prior to etching the plastic substrate with a chrome or chrome-free etchant. It is unexpected that these dilute solutions would have this desirable effect in raising the surface energy of the plastic substrate. In addition, it was surprisingly found the aqueous electrolyte can contain a concentration of the organic salt of less than about 20%. Additionally, it was found that at concentrations of less than about 60%, the aqueous electrolyte solution is very easy to rinse from the plastic surface and thus short rinse times can be utilized.

Once the plastic substrate has been contacted with the pretreating composition, it is then contacted with an etch solution as described in related Pat. Pub. No. 2013/0186774 to Pearson et al., the subject matter of which is herein incorporated by reference in its entirety. By using the process described herein, very high adhesion values can be obtained between the underlying plastic substrate and the metal layer that is plated thereon.

Thereafter, additional steps may be undertaken to obtain a sufficiently adherent metalized plastic substrate.

It should also be understood that the following claims are intended to cover all of the generic and specific features of the invention described herein and all statements of the scope of the invention that as a matter of language might fall therebetween. 

1. A method of treating a plastic substrate to accept metal plating thereon, the method comprising the steps of: a) pretreating the plastic substrate by contacting the plastic substrate with an aqueous electrolyte comprising an organic salt; and thereafter b) etching the treated plastic substrate with an etchant.
 2. The method according to claim 1, wherein the etchant is a chrome based etchant.
 3. The method according to claim 1, wherein the etchant is chrome-free.
 4. The method according to claim 1, wherein the organic salt comprises an ionic liquid and combinations thereof.
 5. The method according to claim 4, wherein the ionic liquid comprises an imidazolium compound having the formula:

wherein R1 and R3 are each, independently of one another, an organic radical having from 1 to 20 carbon atoms, R2, R4 and R5 are each, independently of one another, an H atom or an organic radical having from 1 to 20 carbon atoms, X is an anion and n is 1, 2 or
 3. 6. The method according to claim 5, wherein the imidazolium compound is selected from the group consisting of 1,3-dimethylimidazolium methylsulfate, 1,3-dimethylimidazolium hydrogensulfate, 1,3-dimethylimidazolium dimethylphosphate, 1,3-dimethylimidazolium acetate, 1-ethyl-3-methylimidazolium methylsulfate, 1-ethyl-3-methylimidazolium hydrogensulfate, 1-ethyl-3-methylimidazolium thiocyanate, 1-ethyl-3-methylimidazolium acetate, 1-ethyl-3-methylimidazolium methanesulfonate, 1-ethyl-3-methylimidazolium diethylphosphate, 1-(1-butyl)-3-methylimidazolium methylsulfate, 1-(1-butyl)-3-methylimidazolium hydrogensulfate, 1-(1-butyl)-3-methylimidazolium thiocyanate, 1-(1-butyl)-3-methylimidazolium acetate, 1-(1-butyl)-3-methylimidazolium methanesulfonate, 1-(1-do decyl)-3-methylimidazolium methylsulfate, 1-(1-dodecyl)-3-methylimidazolium hydrogensulfate, 1-(1-tetradecyl)-3-methylimidazolium methylsulfate, 1-(1-tetradecyl)-3-methylimidazolium hydrogensulfate, 1-(1-hexadecyl)-3-methylimidazolium methylsulfate or 1-(1-hexadecyl)-3-methylimidazolium hydrogensulfate, and combinations of one or more of the foregoing.
 7. The method according to claim 4, wherein the ionic liquid comprises 2-hydroxyethylammonium formate or methyltributylammonium methylsulfate.
 8. The method according to claim 6 or 7, wherein the ionic liquid comprises at least one of 1-ethyl-2-methylimidazolium acetate and methyltributylammonium methylsulfate.
 9. The method according to claim 1, wherein the aqueous electrolyte comprises less than 50% by weight of the organic salt.
 10. The method according to claim 9, wherein the aqueous electrolyte comprises less than 30% by weight of the organic salt.
 11. The method according to claim 10, wherein the aqueous electrolyte comprises less than about 20% by weight of the organic salt.
 12. The method according to claim 1, further comprising the step of degreasing and cleaning the plastic substrate prior to step a).
 13. The method according to claim 1, wherein the plastic substrate comprises an acrylonitrile-butadiene-styrene copolymer of acrylonitrile-butadiene-styrene-polycarbonate.
 14. The method according to claim 1, wherein the plastic substrate is contacted with the aqueous electrolyte by spraying or by immersion.
 15. The method according to claim 1, wherein the aqueous electrolyte is maintained at a temperature of between 0° C. and about 120° C.
 16. The method according to claim 1, wherein the plastic substrate is contacted with the aqueous electrolyte for a period of time sufficient to raise the surface energy of the plastic substrate.
 17. The method according to claim 16, wherein the plastic substrate is contacted with the aqueous electrolyte for between about 30 seconds and about 10 minutes.
 18. The method according to claim 1, wherein more than 4 molecules of water per ion of organic salt are available for solvation.
 19. The method according to claim 18, wherein more than 9 molecules of water per ion of organic salt are available for solvation.
 20. The method according to claim 19, wherein more than 10 molecules of water per ion of organic salt are available for solvation.
 21. The method according to claim 1, further comprising the step of rinsing the substrate after step a) and before step b).
 22. The method according to claim 21, wherein the rinsing step is carried out for between about 30 seconds and about 5 minutes.
 23. The method according to claim 22, wherein the rinsing step is carried out for about 1 minute to about 3 minutes. 